Background

In the United States, radiation boost to the tumor bed is routinely delivered as part of treatment for early-stage breast cancer. The EORTC boost study (Bartelink H, J Clin Oncol. 2007 Jun 18) showed that there was a local control benefit to the boost compared to whole breast radiation without boost: 10 year local recurrence 6% vs 10%; however, there was also an increase in severe fibrosis with the boost (4.4% vs. 1.65).

The risk of severe fibrosis is related to the size of the cavity and treatment techniques. According to Berger et al. (IJROBP 2004), for each 100 cc increase in irradiated boost volume, a 4-fold increase in fibrosis may be observed.

The purpose of this study was to evaluate the role of repeat CT simulation for breast cancer tumor bed boost planning in patients who were initially ineligible for electron boost treatment due to cavity size or depth.

The hypothesis was that patients who are ineligible for electron boost treatment based on initial CT for whole breast RT planning might benefit from repeat CT for boost planning through decreased target volume.

The goal was to quantify the benefit of repeat CT simulation for lumpectomy cavity boost planning in these patients.

Materials and Methods

Out of 80 patients who were treated at Boston Medical Center from October 2010-September 2011, 20 patients were ineligible for electron boost treatment due to lumpectomy cavity (LC) maximum depth > 5 cm on CT simulation (CT1).

These twenty patients then underwent repeat simulation (CT2) for boost planning at 40.0±5.2 Gy (median 45 days from CT1; range, 21 to 71

The boost prescription dose was 10 or 14 Gy in 2 Gy fractions following a course of 50 or 50.4 Gy to the whole breast.

LC volumes and maximum depths were compared between the two scans.

CT1 boost plans were transferred to CT2 and then compared with CT2 boost plans to assess treated volume and technique changes upon repeat CT simulation.

Pearson correlation coefficients were used to identify parameters that predict a dosimetric advantage of repeat CT; a p-value of ?0.05 was considered statistically significant.

Results

The mean LC maximum depths based on CT1 and CT2 were 6.4±1.3 cm and 5.2±1.3 cm, respectively (p =0.0002).

Volume encompassed by the boost plan prescription isodose surface (V100%) was reduced in 18/20 (90%) patients after re-planning. V100% was 183.6±142.0 cc based on CT2 plans and 320.6±236.6 cc based on CT1 boost plans (p =0.008).

While all 20 patients (100%) required a photon mini-tangential boost technique based on CT1, 8 patients (40%) became eligible for electron boost treatment after re-simulation.

These 8 patients had a 2.7-fold greater absolute reduction in the mean V100% compared to patients treated with mini-tangents. Six of 8(75%) of patients had a mean V100% reduction greater than 100 cc. This difference in mean V100% reduction was further pronounced, 4.5-fold, in 10 patients who experienced an increased LC volume on CT1 relative to pathologic surgical specimen volume.

Author's Conclusions

In breast cancer patients with large lumpectomy cavities and maximum lumpectomy cavity depths > 5 cm on initial CT simulation, tumor bed boost planning based on a newly acquired CT towards the end of the whole breast irradiation course is associated with significant treatment volume reduction and may result in a change of treatment technique.

This has the potential to improve cosmesis, a clinically important outcome in breast-conserving therapy.

Clinical Implications

This is an important study that helps to identify patients initially ineligible for an electron boost who can then receive a electron boost after resimulation. Clinically, this is a sound study and can provide clinical practice guidelines that for patients with a maximum lumpectomy cavity depths > 5 cm on initial CT simulation, resimulation should be considered.

Many studies have shown that the tumor bed decreases in size over the course of radiotherapy. Sharma et al. (IJROBP 2008) showed that Length of time from surgery to start of radiation therapy showed an inverse correlation with change in seroma volume (Pearson correlation r = -0.53, p < 0.01). Hezel et al. (IJROBP 2009) showed that on comparison of the tumor bed volume from the initial planning CT scan to the boost planning CT scan, there was a decrease in size in 77% of cases. The mean decrease in volume was 52%. The study here, however, is the first to demonstrate that electrons can be used instead of photons in many cases if resimulation is used.

One limitation of this study is that it does not evaluate local control. In decreasing the size of the lumpectomy cavity, future studies will have to assure that the improvement in local control seen in the EORTC boost trial is not lost.

For patients with large lumpectomy cavities inappropriate for electron therapy even on re-simulation, other techniques can be considered including brachytherapy and proton beam therapy.